Abstract: The underground-harmful-gas is one of the typical natural hazard of metro construction. In the engineering practice, the best way is to exhaust the harmful gas in advance. In this article, we will show you a way on how to survey on underground harmful gas which based on our engineering practice and statistics, so we can have the accurate information about the burial depth, distribution range, components and variation of the harmful gas. Besides, we will propose the measures can taken to tackle the problem in construction and later stages of construcion to ensure the Project can carry smoothly.